The DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT)mediates resistance to BCNU and related alkylating agents in human tumors by removing O6-alkylguanine DNA adducts produced by these drugs. Inibitors of AGT activity, such as O6-benzylguanine (BG), increase sensitivity to these alkylating drugs and are potentially important modulators of several common anti-neoplastic drugs. One limitation of BG is that its AGT-inhibiting activity is non-specific so that normal cells and tissues in addition to tumors, are sensitized to DNA damaging agents. The central hypothesis of this program is that the 9-substituted derivatives of BG will be more effective than BG because of more favorable pharmacologic characteristics and relatively selective activation in certain tissues and tumors. Preliminary data indicate that two such analogs, O6-benzyl-2'- deoxyguanosine and O6-benzylguanosine, are effective in inhibiting AGT in human tumor xenografts even though they are substantially less potent AGT inhibitors than BG in vitro. Our data support the idea that this enhanced AGT inhibition in both rats and human brain tumor xenografts in athymic mice is due to greater systemic availability of the parent compound and, more importantly, to site-specific activation of the parent compound to BG and O6-benzyl-8-oxo-guanine, which are two metabolites with considerable AGT-inhibiting activity. These data encourage additional screening of 9- substituted BGs to identify more potent and more site specific AGT inhibitors and to establish the mechanism of their enhanced inhibitory activity in vivo. We also propose to optimize the use of these inhibitors by adjusting doses and administration schedules. Our aims are to; 1) determine the extent and duration of AGT suppression in human brain tumor xenografts and normal organs in tumor-bearing athymic mice after administration of 9-substituted BGs; 2) identify the principal metabolites of 9-substituted BGs in rats, athymic mice, and human tumor xenografts; 3) determine the relationship between the pharmacokinetics and metabolism of 9-substituted BGs and the extent and duration of AGT inhibition in brain tumor xenografts and normal organs, and 4) optimize the BCNU potentiating effects of the 9-substituted BGs against human brain tumor xenografts. We expect to identify potent AGT inhibitors with enhanced therapeutic indices and selectivity against brain neoplasms.